1. Field of the Invention
This invention relates to latent foamable compositions for use in or as adhesives, sealants and/or coatings. The compositions include a curable component or a thermoplastic component, together with a latent foaming agent. The latent foaming agent is capable of ambient temperature performance when exposed to UV radiation, or elevated temperature performance. In curable versions of the inventive compositions, a cure initiator or catalyst may also be included.
2. Brief Description of Related Technology
The commercial marketplace for adhesives, coatings and sealants, which are cured by exposure to UV or visible light radiation, is a rapidly growing segment of the adhesives industry. The advantages of such UV or visible light curing include providing products with a larger percentage of reactive materials, on-demand curability, fast curability, minimized opportunity to compromise the integrity of the parts to be bonded or sealed, and lower energy consumption, as contrasted to conventional heat cure processing.
Perhaps the most widely used UV or visible light curing formulations, which are initiated by free radicals, include generally mono- and di-functional (meth)acrylate monomers, a photoinitiator, and optionally, an acrylate-functional pre-polymer. The phototinitiator generates free radicals on exposure to UV or visible light that initiate cure. However, (meth)acrylate based formulations are known to possess certain limitations generally, including oxygen inhibition of (meth)acrylate polymerization at the air/coating interface, limited cure through depth, and difficulty in curing highly pigmented systems.
Various methods have been studied to overcome or minimize oxygen inhibition at the coating/air interface, such as the use of high intensity light, dual-cure mechanisms, or placing a physical barrier over the coating.
The use of o-acylthiohydroxamates has been reported to reduce the tack free time of UV curable coatings, to which they have been added. Indeed, in U.S. Pat. No. 4,954,415 (Davis), a photohardenable composition including a free radical addition polymerizable material and a photoinitiator composition is claimed. The photoinitiator composition includes a compound, which absorbs actinic radiation and directly or indirectly generates free radicals, and an o-acylthiohydroxamate or an N-alkoxypyridinethione. The compositions described in the '415 patent were developed for and are reported as useful in photoimaging applications.
o-Acylthiohydroxamates generally decompose on exposure to UV radiation to provide radicals, which can either recombine or initiate acrylate polymerization at the surface to overcome oxygen inhibition, as seen below in eq. 1. The major by-product of the reaction is an alkylpyridyl sulfide, 2. 
To date, however, it has not been recognized or reported that such materials may form the basis for a foaming system, particularly for use in a plastic matrix.
Rather, creating foamed polymers generally involves the use of for example a blowing agent, which yields a gas on thermal exposure, the production of carbon dioxide by the reaction of a diisocyanate with water, or the generation of a foam by the addition of a gas or low boiling liquid to a molten polymer. To that end, many foamable compositions are known. See e.g. U.S. Pat. Nos. 4,686,244 (Dietlin) and 5,246,973 (Nakamura). See also U.S. Pat. Nos. 4,808,634 (curable foaming silicone composition containing a vinyl polysiloxane, a hydride polysiloxane, a hydroxyl source selected from organic alcohol or organic alcohol in combination with water or hydroxylated organosiloxane, from 1 to about 250 ppm of platinum catalyst and a ketoximine compound effective to lower the foam density. Foam is created through the reaction of the hydride polysiloxane with the hydroxy source to liberate hydrogen gas.); 5,358,975 (organosiloxane elastomeric foams, incorpoarting a triorganosiloxy end-blocked polydiorgano siloxane, an organohydrogen siloxane, a platinum catalyst, an α, β, ω-diol and a resinous copolymer containing siloxy vinyl groups. Foaming is produced as a result of the reaction of the polyhydrogen siloxane and the alcohol which liberate hydrogen gas.); 6,110,982 and 5,373,027 (use of blowing agents to effectuate foams); 6,207,730 B1 (epoxy composition to which is added thermoplastic shell microspheres, which may encapsulate a gas.); 6,277,898 B1 (epoxy resins useful as photocurable paints which use chemical or mechanical expansion agents to create foams.); and 5,356,940 (fine pored silicone foam which is formed by mixing a vinyl silicone, an organo-platinum catalyst, fumed silica, and water as a first part, with a second part which includes a silicone polymer having at least two double bonds per molecule, fumed silica and polydimethylhydrogensiloxane. The two components are mixed and the reaction is subsequently pressurized, using air or nitrogen, so that the pressurized gas is present in the mixture in a dissolved form. Subsequently, the reaction mixture is heated and the dissolved gas is released, thereby forming a fine pored silicone foam.); 5,900,430 (silicone foaming compositions which contain an organopolysiloxane containing a specified amount of an alkenyl group and/or a hydroxyl group, an organohydrogenpolysiloxane, a compound having an active hydrogen, such as an alcohol, a platinum catalyst and an acetylenic alcohol compound. Foaming occurs during the cure process by the reaction of the compound having an active hydrogen group, i.e., an alcohol, with the organohydrogenpolysiloxane compound to release hydrogen gas.); 5,061,736 (foamable silicone compositions useful as fire-resistant joint-sealing members. The compositions disclosed contain a diorganopolysiloxane, a finely divided reinforcing silica filler, a powder of a ferrite such as a divalent metallic element such as manganese, copper, nickel, from 10 to 70 parts by weight of a finely divided inorganic material, such as mica or glass powders, finely divided platinum metal, a curing agent for silicone rubbers such as a peroxide or condensation catalyst, and the blowing agent is a composition which produces a foaming gas when exposed to elevated temperature, such as nitrogen, carbon dioxide, azobisiobutyronitrile.); 6,003,274 (reinforcement web for a hollow structural member having layer of expandable foam—a resin-based material containing a blowing agent—dispersed on its principal surfaces.); 5,575,526 and 6,092,864 (laminates which have support members or beams bonded together with a structural foam layer, produced by a synthetic structural resin combined with a cell-forming agent (blowing agent) and hollow microspheres); 6,218,442 B1 (corrosion-resistant foam formulation which includes one or more thermosettable synthetic resins, one or more curatives, one or more blowing agents and one or more organic titanates or zirconates.)
Notwithstanding the state-of-the-art, it would be desirable to provide a latent foaming agent for use with a matrix—whether or not curable—, which matrix is destined for use in applications where foaming or dimensional expansion may be desirable.